Object Data Model · CSIT115 Data Management and Security Object Data Model Dr Janusz R. Getta...

CSIT115 Data Management and Security

Object Data ModelDr Janusz R. Getta

School of Computing and Information Technology -University of Wollongong

Object Data Model file:///Users/jrg/115-2020-SUMMER/LECTURES/03objectdatamodel/03objectdatamodel.html#1

1 of 45 21/11/20, 10:49 pm

Object Data ModelOutline

Graphical Notations for Conceptual Modeling

(Simplified) Class of Objects

Association

Link Attribute

Association Class

Qualification

Generalization

TOP Created by Janusz R. Getta, CSIT115/CSIT815 Data Management and Security, Summer 2020/2021 2/45


2 of 45 21/11/20, 10:49 pm


Entity-Relationship (ER) diagrams (1976)



3 of 45 21/11/20, 10:49 pm


Object Modelling Technique (OMT) diagrams (1991)



4 of 45 21/11/20, 10:49 pm


(Simplified) Unified Modelling Language (UML) Object Class diagrams(1994)



5 of 45 21/11/20, 10:49 pm


Object Role Modeling (ORM) diagrams

... and many, many, ... other graphical notations



6 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



7 of 45 21/11/20, 10:49 pm


A database suppose to contain information about people

A rectangular box with a header and a name of class (PERSON) inside aheader represents a (simplified) class of objects

A person is described by a social security number, name, date of birthand address

The names of attributes are listed within a rectangular box one name ofattribute per row



8 of 45 21/11/20, 10:49 pm


A person has from one to five email addreses and zero or more phonenumbers

Multiplicity of attribute like [1..5] (from one to five), zero or more ([*]or [0..*]), one or more ([1..*]), optional (zero or one, [0..1]), from"m" to "n" ([m..n]) follows a name of attribute

Default multiplicity is "one" ([1])



9 of 45 21/11/20, 10:49 pm


A person is described by an optional country of origin and age (/ in frontof attribute name age denotes a derived attribute)



10 of 45 21/11/20, 10:49 pm


A person is identified by a social security number and independently bya triple (name, date of birth, address)

A tag IDx following a name of attribute means, that an attribute is anidentifier

If the same tag (e.g. ID2) follows the names of several attributes then itmeans that an identifier consists of several attributes

If an identifier consists of several attributes then it means, that eachobject in a class is identified by a tuple of values of the attributes

An identifier which consists of several attributes is called as a compositeidentifer



11 of 45 21/11/20, 10:49 pm


A vehicle is described by a registration number, manufacturer, model,year when manufactured and optional fuel consumption

Each vehicle has a different registration number



12 of 45 21/11/20, 10:49 pm


Summary



13 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



14 of 45 21/11/20, 10:49 pm

Association

A person owns a vehicle

A solid line that connects two classes represents an association

A name above a line is an association name

An association name if followed (preceeded or located above or below)by a small solid triangle, that represents a direction of interpretation ofan association



15 of 45 21/11/20, 10:49 pm

Association

A vehicle has an owner

Locations of classes and associations in a diagram are immaterial



16 of 45 21/11/20, 10:49 pm

Association

A person owns zero or more vehicles

A vehicle has one owner

Multiplicities of association are located at both ends of a line thatrepresents an association



17 of 45 21/11/20, 10:49 pm

Association

One-to-one associationA department has a manager

A department has exactly one manager

A person who is a manager manages exactly one department

-

-

-



18 of 45 21/11/20, 10:49 pm

Association

One-to-many associationA company employs an employee

A company employes many employees

An employee works for none or one company

It is possible that a company has no (zero) employees

-

-

-

-



19 of 45 21/11/20, 10:49 pm

Association

Many-to-many associationA supplier supplies a part

A supplier supplies zero or more parts

A part is supplied by zero or more suppliers

-

-

-



20 of 45 21/11/20, 10:49 pm

Association

Summary



21 of 45 21/11/20, 10:49 pm

Association

More examples:

A team consists of 22 players

A player belongs to only one team

A game is played by 11players plus maximum 3 substitutes

A player participates in zero or more games



22 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



23 of 45 21/11/20, 10:49 pm

Link Attribute

A link attribute is an attribute that describes an association

A student enrols a subject

An enrolment is performed on a given day, it means, that an enrolmentis described by an enrolment date

A link attribute is graphically represented by a "class-like" rectangularbox without a name of class in a header



24 of 45 21/11/20, 10:49 pm

Link Attribute

Why an atribute enrolment-date must be a represented by a linkattribute ?

For example, what about an attribute enrolment-date describing aclass STUDENT ?

Such design is incorrect because information about which subject hasbeen enrolled by a student and on what date is missing

A student is associated with a set of enrolment-dates, however ...

... there is no link between the values of an attribute enrolment-dateand the objects in a class SUBJECT



25 of 45 21/11/20, 10:49 pm

Link Attribute

So what about another option where an atribute enrolment-datedescribes a class SUBJECT ?

Such design is also incorrect because information about who enrolled ina subject and on what date is missing again

A subject is associated with a set of enrolment-dates, however ...

... there is no link between the values of an attribute enrolment-dateand the objects in a class STUDENT

Therefore, the only option for an attribute enrolment-date is to be alink attribute that describes an asociation between a class STUDENT anda class SUBJECT



26 of 45 21/11/20, 10:49 pm

Link Attribute

If an attribute enrolment-date describes an association Enrolsbetween the classes STUDENT and SUBJECT then its values are likelabels attached to attached to the links between the objects from theclasses STUDENT and SUBJECT



27 of 45 21/11/20, 10:49 pm

Link Attribute

Another example of link attribute

A supplier supplies a part

A shipment of a part is performed on a given day, it means, that ashipment is described by an attribute shipment date

A shipment contains a given number of parts and it means, that ashipment is also described by an attribute quantity



28 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



29 of 45 21/11/20, 10:49 pm

Association Class

An association class is a class that represents an association

A student enrols in a subject

An enrolment is performed on a given day, it means, that an enrolmentis described by an attribute enrolment-date

An association class ENROLMENT is graphically represented in the sameway as a class of objects



30 of 45 21/11/20, 10:49 pm

Association Class

What about an identifier of an association class ?

An association class may have its own identifiers, for exampleenrolment-number in a class ENROLMENT

An association class that represents many-to-many association has adefault identifier that consists of identifiers of the classes from bothsides of association, for example a pair (snumber, code) in a classENROLMENT

There is no need to include a default identifier into a description of aclass of objects



31 of 45 21/11/20, 10:49 pm

Association Class

Another example of association class

A supplier supplies a part

A shipment of a part is performed on a given day, it means, that ashipment is described by an attribute shipment-date

A shipment contains a given number of parts, it means, that a shipmentis also described by an attribute quantity



32 of 45 21/11/20, 10:49 pm

Association Class

What can we use association classes for ?

A shipment is delivered to a customer

An association Delivered-to connects the classes SHIPMENT andCUSTOMER

In a general case, it is possible to create associations betweenassociation classes and other classes of objects and between associationclasses and association classes



33 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



34 of 45 21/11/20, 10:49 pm

Qualification

A building at a university campus is described by a unique number,optional name and the total number of floors

A room is described by a number and area

A building consists of rooms

What is an identifier of a class ROOM ?

Identifier of a class ROOM is a composite identifier and it consists of theattributes bnumber from a class BUILDING and rnumber from a classROOM

How do we represent composite identifiers that consist of attributesfrom more than one class ?

We use a qualification !



35 of 45 21/11/20, 10:49 pm

Qualification

A building at a university campus is described by a unique number,optional name and the total number of floors. A room is described by anumber and area. A building consists of rooms

A qualification is represented by a rectangle with one or more attributenames

In the example above a qualification means that in a given building thereis at most one room with a given number

It also means that an attribute rnumber is a local identifier in a classROOM, i.e. all rooms in a given building have different numbers

A pair of attributes (bnumber, rnumber) is a default identifier of aclass ROOM



36 of 45 21/11/20, 10:49 pm

Qualification

More examples:

A bank account is located at a bank

A campus consists of buildings and buildings consist of rooms

A company is listed at stock exchange



37 of 45 21/11/20, 10:49 pm

Qualification

Sometimes qualification is needed for many-to-many associations

For example, if we consider a pessimistic view of students who enrol insubjects then sometimes a student must enrol in the same subject morethan one time (well, ... you know when it happens and why it is a"pessimistic" view of reality)

Some time ago we created the following design

But, ... if enrolment is repeated several times then a pair of attributes(snumber, code) is no longer an identifier of a class ENROLMENT !



38 of 45 21/11/20, 10:49 pm

Qualification

If enrolment is repeated several times then an identifier of a classENROLMENT is a triple (snumber, code, enrolment-date)

How is this represented graphically ?

It is represented by a qualification of the middle part of the associationEnrols

Qualification of the middle part of many-to-many association means,that an identifier of association class consists of an identifier of a classon the left hand side of association, identifier of a class on the right handside of association, and qualification attribute(s)



39 of 45 21/11/20, 10:49 pm

Qualification

A qualification of the middle part of many-to-many-association isequivalent to the following two single-side qualifications of one-to-manyassociation and many-to-one association



40 of 45 21/11/20, 10:49 pm




Association

Link Attribute

Association Class

Qualification

Generalization



41 of 45 21/11/20, 10:49 pm

Generalization

A generalization hierarchy represents Is-a-subset relation between theclasses of objects

A set of all undergraduate students is a subset of a set of students, itmeans, that an undergraduate student IS-A student

A set of all postgraduate students is a subset of a set of students, itmeans, that a postgraduate student IS-A student

A generalization hierarchy is built with arrows pointing from a subclassto a superclass



42 of 45 21/11/20, 10:49 pm

Generalization

Another graphical notation for generalization hierarchy:

A car IS-A vehicle and a truck IS-A vehicle

To create a nice effect the arrows can be "joined" into a single arrow



43 of 45 21/11/20, 10:49 pm

Entity-Relationship Modeling

Entity-Relationship Modeling is another graphical conceptual modelingnotation

It is presented in your texbook

Graphically it is very similar to Object Data Model

The following concepts from the notations are equivalent:Class of objects = Entity type

Object = Entity instance

Association = Relationship

Identifier = Primary key or Candidate key

Qualification = Weak entity type

-

-

-

-

-



44 of 45 21/11/20, 10:49 pm

References

C. Coronel, S. Morris, A. Basta, M. Zgola, Data Management and Security,Chapters 2, 3, and 4 Cengage Compose eBook, 2018, eBook: DataManagement and Security, 1st Edition

T. Connoly, C. Begg, Database Systems, A Practical Approach to Design,Implementation, and Management, Chapter 12 Entity-RelationshipModeling, Chapter 13.1 Specialization/Generalization, Pearson EducationLtd, 2015



45 of 45 21/11/20, 10:49 pm

Object Data Model · CSIT115 Data Management and Security Object Data Model Dr Janusz R. Getta...

Documents

Transcript of Object Data Model · CSIT115 Data Management and Security Object Data Model Dr Janusz R. Getta...